1. Field
One or more aspects of embodiments according to the present invention relate to inverse synthetic aperture radar (ISAR) and more particularly to a system and method for obtaining profile presentation images of a vessel imaged using an ISAR system.
2. Description of Related Art
Inverse synthetic aperture radar (ISAR) is a technique used to obtain high resolution radar images of a moving target, and in particular of a periodically rotating target. The resolution may be superior to the diffraction-limited resolution of the physical aperture used because, in an ISAR system, a synthetic aperture may be formed, by combining radar returns received from the target at various target angles, that is significantly larger than the physical aperture.
An ISAR system may use the Doppler shift of the return signals received from points on a target to infer the range velocity of the points, from which, assuming that the target is rotating as a rigid body, the relative distance of points from the axis of rotation, in a direction perpendicular to the radar beam may be inferred. Using this method, ISAR may be used to distinguish different types of vessels at sea. A military vessel, for example, may have structures on its deck that differ from those ordinarily found on a commercial vessel. These structures will be most easily discerned by a conventional ISAR system when the ISAR image is a profile presentation. In this presentation, if the vessel is rolling or pitching, a point high above the deck, e.g., on a mast, and far from the instantaneous axis of rotation, may produce a return with a greater Doppler shift than a point near the axis of rotation, e.g., the roll axis of the vessel. The rotational motion of the vessel may be approximately periodic and sinusoidal, e.g., as the pitches alternately fore and aft, and as a result there may be points in time at which the pitch or roll rate is small, when the periodic rotation rate changes direction. At these points in time a conventional ISAR may be poorly able to distinguish, e.g., a point high above the deck from a point near the axis of rotation because the range rate of both points will be nearly the same.
The spectrum of waves and swells in an ocean results in nearly periodic rolling and pitching motion for vessels, with a period of as long as approximately 10 seconds; some yaw motion may be present also. Generally, pitch or roll motion tends to produce the desirable profile target presentation; yaw produces the less desirable—but still useful—plan view (i.e., top view) of the target. A conventional ISAR system may present to an operator a series of frames corresponding to consecutive ISAR images taken of the target. The operator may inspect each image, discard those that are not helpful for identifying the target, e.g. ones corresponding to images with little instantaneous rotational motion, and save those that are helpful, i.e., those that are nearly profile presentations. A conventional ISAR system may therefore allocate 10 seconds to each contact, to provide a high likelihood that the operator will be presented with at least one image from which a reliable identification can be made. This allocation is impractical for automated systems that require rapid target identification, such as automatic target recognition (ATR) systems, which may, for example, be employed in weapons targeting systems. Thus, there is a need for an ISAR system capable of more rapid target recognition.